Banwell Ratio

The Banwell Ratio is described in Colin Rogers, ‘The Surname Detective’ (1995) pp.21-22: “The aim is to obtain a figure for each surname within a specificed … area which will indicate how far it is above or below the number which you would expect from an even distribution”.

It can be calculated as follows:

Let X be the number of occurrences of the surname in the selected county, divided by the total population of that county.

Let Y be the total number of occurrences of the surname in the census, divided by the total census population.

Then the Banwell number is X divided by Y.

For the Kent row in the table above (for the surname Waghorn), this gives:

X

= 637 / 977,517

= 0.000652

Y

= 1,169 / 29,833,812

= 0.0000392

Banwell

= 0.000652 / 0.0000392

= 16.63

“The result will vary from 0.0 in areas which have no instances of that surname, through 0.5 (in areas where you find only half the number of surnames compared with the figure you would expect if they were spread evenly thoughout the population), 1.0 (average), 2.0 (twice the number expected from a random distribution), 3.0 (three times) and so on” (Rogers, p.22)